A high pressure pump is conventionally known in the art, according to which liquid fuel is supplied by a low pressure pump from a fuel tank to the high pressure via a fuel line, the fuel is pressurized by a reciprocal movement of a plunger operated by a rotation of a cam shaft, and such pressurized fuel is pumped out by the high pressure pump to a side of fuel injectors.
A pumping operation of the high pressure pump for pressurizing the fuel is composed of a suction stroke for sucking the fuel from a fuel gallery of the pump into a fuel pressurizing chamber when the plunger is moved from its top dead center toward a bottom dead center, a fuel amount adjusting stroke for returning a part of the fuel to the fuel gallery when the piston is moved from the bottom dead center toward the top dead center, and a pressurizing stroke for pressurizing the fuel when the plunger is further moved toward the top dead center after a suction valve is closed.
During the fuel amount adjusting stroke, that is, an initial stage of an upward movement of the plunger, a part of the fuel is returned from the fuel pressurizing chamber into the fuel gallery formed on a fuel suction side, so that the fuel amount to be pumped out from the high pressure pump to the side of the fuel injectors is controlled. During such fuel return, pressure pulsation may be generated in the fuel gallery. The pressure pulsation may be propagated to a fuel pipe for supplying fuel from the low pressure pump to the high pressure pump, or to the low pressure pump. As a result, unnecessary vibration and/or noises may be generated. In a conventional high pressure pump, for example, a pulsation damper is provided in the fuel gallery in order to decrease the pressure pulsation.
According to a prior art high pressure pump, for example, as disclosed in International Patent Publication No. 2008-525713 (published in Japan), a volume compensating chamber and a pressure attenuating device are described in order to decrease the pressure pulsation, for example, in paragraphs [0021] and [0022] thereof. According to this prior art, an annular stepped portion 48 defines a volume compensating chamber 94 which is separated from a working chamber 38, and the volume compensating chamber 94 is communicated to a space for a volume control valve 26 on a fuel suction side which is separated from the working chamber 38.
According to another prior art, for example, as disclosed in Japanese Patent Publication No. 2008-002361, a high pressure pump has a plunger with a large-diameter portion and a small-diameter portion.
According to a further prior art, for example, as disclosed in Japanese Patent Publication No. 2008-286144 (corresponding to US 2008/0289713 A1), fuel from a fuel inlet is drawn by reciprocating of a plunger into a fuel pressurizing chamber through an intake valve mechanism, which is provided at an inlet of the fuel pressurizing chamber. The fuel pressurized in the fuel pressurizing chamber is discharged through an expelling valve mechanism, which is provided at an outlet of the fuel pressurizing chamber. A damper housing part is disposed at an intermediate point of a fuel channel between the fuel inlet and the intake valve mechanism.
According to the above prior arts, a variable volume chamber, which is formed at an opposite side of a plunger to a fuel pressurizing chamber, is communicated to a fuel gallery. And a pulsation damper having a specified shape is accommodated in the fuel gallery communicated with a suction valve, so as to absorb pressure pulsation.
However, the above prior arts disclose nothing more than the pulsation damper in the fuel gallery in order to decrease the pressure pulsation. According to the prior arts, the pressure pulsation is decreased to some extent. It is, however, not a satisfactory level.
According to the above prior arts, the variable volume chamber is communicated with the fuel gallery. Since the fuel pressurizing chamber and the variable volume chamber are defined by the same plunger, the volume changes take place in phase, that is, in a conjugated relation. The variable volume chamber is used in a supplementary manner when the fuel is sucked into and/or discharged from the fuel pressurizing chamber through the fuel gallery.
Since apart (a small-diameter portion) of the plunger extends through the variable volume chamber, the volume change of the variable volume chamber is not the same to that in the reversed phase of the fuel pressurizing chamber. Therefore, the variable volume chamber may play only a supplementary role in absorbing excessive amount of the fuel from the fuel pressurizing chamber. In other words, the variable volume chamber can not absorb 100% of the excessive amount of the fuel from the fuel pressurizing chamber during the fuel amount adjusting stroke.